HONG KONG — Protesters, police officers and lawmakers converged on Hong Kong’s Legislative Council on Wednesday as debate began on a plan to change the way the semiautonomous Chinese territory will choose its leader beginning in 2017.

Supporters of the new plan say that it would dramatically expand voters’ say over who governs their city. Opponents, including the 27 members of the legislature’s pro-democracy camp, say that restrictions on who can appear on the ballot mean the proposed system falls short of “true” universal suffrage. They plan to reject the plan, which needs the support of at least 47 of the legislature’s 70 members to pass.

The security presence in and around the Legislative Council was high on Wednesday after heated protests last year over the proposed changes. Demonstrators occupied the area around the government complex and major roadways for several weeks last year. A small protest camp has remained outside the legislative building since.

Hundreds of pro-government demonstrators, including many older people, gathered outside the council building on Wednesday. They were organized by neighborhood associations and were equipped for the day’s blistering sun with hats and water bottles. A few opponents of the bill stood mostly on the periphery and were closely watched by the police.

The lawmakers’ debate, which could involve more than 17 hours of speeches if each of the 70 members uses the allotted 15 minutes of speaking time, was carried live on the Legislative Council’s website, with simultaneous English and Mandarin interpretation.

The measure is expected to be defeated, but even its opponents recognize that a vote that goes their way is no victory.

“I opened my computer this morning and found a long, very well-written letter from a student,” Ronny Tong, a moderate from the Civic Party who is likely to vote against the proposal, said on Wednesday before the debate began. “The student hopes that I could support the proposal.”

“Everybody is a loser today,” Mr. Tong said. “I just hope it’s not a total loss. I hope we will have the chance to fight in the legislature for electoral reform that people can accept.”

Top government officials made last-ditch efforts on Wednesday to try to persuade pro-democracy legislators to change their minds.

“My team and I have worked on this for 20 months,” said Carrie Lam, the No. 2 official in the Hong Kong government, recounting more than 100 town hall-style meetings and news conferences. “We have come against much criticism, mockery and even insults.”

“We have made our best effort and come up with the best and most practical proposal,” she said. “It’s always better to have a vote in your hand than nothing, and progress to universal suffrage is better than a standstill.”

Alan Wong and Michael Forsythe contributed reporting.

A version of this article appears in print on , on Page A6 of the New York edition with the headline: Election Plan Is Debated in Hong Kong. Order Reprints | Today’s Paper | Subscribe